The present invention relates in general to the protection of electronic equipment from the effects of power line surges, transients and other such electrical disturbances. In particular, the present invention relates to minimizing the let-through voltage associated with any circuits used to protect electronic equipment from the effects of such electrical disturbances.
Communications equipment, computers, automated test and production equipment, military targeting systems, home stereo systems, televisions and other electronic devices as well as electrical loads using integrated circuits, solid state components, semiconductor networks and the like, are increasingly characterized by small electrical contacts and miniature components which are very vulnerable to interference or damage due to interference or damage from electrical disturbances carried by power line conductors connected thereto. Unpredictable variations in power line conductor voltage changes the operating range and can severely damage or destroy such devices. These devices and related process problems are very expensive to repair or to replace and therefore, require cost-effective protection from transients and surges associated with the power.
For convenience of discussion, electrical disturbances may be classified as one of several different types: electrical noise, voltage sags or surges and transients.
Electrical noise, also known as "hash" results from random changes in voltage level. Voltage changes due to noise are not typically very large but they are fairly frequent, such that if a normal operational voltage level were plotted on a graph as a function of time, the voltage level would be represented by a "fuzzy" line instead of a sharp line. Noise in an electrical circuit may be caused by electromagnetic interference from nearby fluorescent lights, transformers, computers, car ignitions, radio and television transmissions, electrical storms or other such sources of electromagnetic or radio-frequency signals.
Voltage surges are a decrease or an increase, respectively, of an AC voltage level for one or more voltage cycles. Sags or surges may originate from many sources, such as loose connections in a device, switches, power overloads, lightning, accidents blackout and brownout corrections, short circuits, grounding or operation of nearby electric motors and generators.
Transients are high-voltage pulses having an extremely fast rise time, typically on the order of a few microseconds, although some transients may last for a period of up to milliseconds or larger. Typically, they may reach a peak voltage of as high as ten thousand volts.
Furthermore, extremely high voltages, mostly due to transients, may additionally cause insulation breakdown, "hot spot" melting of semiconductors, and the destruction of many delicate circuit components which in turn may necessitate costly production losses, service calls and repair time. Additionally, each of these types of electrical disturbances may cause data errors, unscheduled downtime, circuit board failure, transistor failure, disruptive false commands, or the loss of computer memory.
On the other hand, low energy transients may have very fast rise times and peak voltages of ten-thousand volts or higher. Even though the energy is low, the high voltages can damage common household equipment using low-level transistors, such as calculators, radios and the like, and industrial equipment such as computers, instruments, and controls. Higher energy transients may involve energy as high as one-hundred joules and surge currents of as high as ten thousand amperes. Inductive loads, such as motors and transformers, can experience buildup of high instantaneous voltages across their windings from such transients. Resultant arcing across the turns damages the insulation, resulting in shorting out of the turns. Such effects are often progressive, resulting in the ultimate failure of the motor or transformer, or electrical device.
A major manufacturer of appliance motors has analyzed many failures and has determined that over seventy-five per cent were directly or indirectly caused by transients or over-voltage surges. Motors, as well as some of the other above-named equipment, can be designed to withstand surges; however, the extra cost is too great for the consumer appliance market, as well as for many industrial situations.
As discussed above, one source of harmful electrical energy is lightning. Lightning is a very complex electromagnetic energy source having potentials estimated at from five million to twenty million volts and currents reaching thousands of amperes. A lightning stroke generally contains a series of pulses each having a duration of from a nanosecond to several milliseconds. A typical "8/20" lightning pulse lasts for a period of forty microseconds and has a peak current of three thousand to twenty-thousand amperes which is reached in about eight microseconds.
Another source of unwanted electromagnetic energy is a nuclear electromagnetic pulse (EMP or HEMP). An electromagnetic pulse generated by a nuclear detonation produces intense transient electric and magnetic fields with very short rise times and a frequency spectrum extending from approximately zero to more than one hundred megahertz. The electromagnetic pulse from a high altitude explosion typically has a maximum field strength near the ground on the order of fifty kilovolts per meter, a time duration of one microsecond and a rise time of nanoseconds.
Other sources of impulse noise are ground loop interference caused by varying ground potentials.
The above-mentioned noise transient can be coupled to a sensitive electronic device by external or internal coupling. In external coupling, an electromagnetic wave or a lightning pulse impinges on a receiver, such as a power transmission line or system, and the transient voltage induced in the receiver is passed through transformers, rectifiers and other voltage and current altering devices to the electronic devices. The passage of the transient through such devices as transformers alters the original waveform increasing the current levels, the voltage levels and the frequency spectrum. The characteristics of the transient reaching the equipment can be hard to predict because of intermediate coupling devices, thereby making the transient hard to remove.
In general, protection of a system against transient overvoltages by surge protectors requires that these protectors be able to quickly reduce the transient overvoltage to a safe level well below the breakdown voltage of the device being protected without causing a system outage. The primary function of the protecting device is to maintain voltage applied to the protected equipment below that level of voltage which can safely be accepted by such protected equipment. These devices operate on the principle that there effectively is an open circuit through the device until the voltage across the protecting device reaches a critical level, and upon reaching the critical voltage associated with the protecting device, the resistance of the protecting device breaks down to a very low value and the heretofore open circuit breaks down to be a shunt path across the power lines, or to ground. The break down should occur quickly enough to provide protection for advanced electronic devices. There is a voltage and operating speed associated with the device itself, with this voltage sometimes being referred to as the clamping voltage, and thus, these protecting devices are known as clamping elements.
Today, there are many devices that have performance characteristics that adapt them for use as suppression elements. Examples of such devices include special power supply circuits, voltage regulators, motor-generators, constant voltage transformers, gas tubes, capacitors, silicon avalanche suppressors (SAS) and metal oxide varistors (MOV).
While effective, each of these protecting devices has its own drawbacks and is not capable of protecting a device against all of the above-mentioned electrical disturbances.
The fact that no single device can adequately function as a suppressor for all transients that may be encountered has led to the development of what are known in the art as hybrid circuits which attempt to incorporate the beneficial features of a plurality of separate protective devices into one circuit.
While theoretically quite effective, the real-world performance of such suppression devices, whether alone or in a hybrid circuit, vis a vis the theoretical performance thereof often differs. It is the real-world performance of a suppressor that is important to the designer and the user of such an element.
The true test of the real world performance of a suppression device is how low and how quickly the voltage across the device being protected is held during a transient surge. This voltage is the voltage to which the protected device will be exposed. For the purposes of this disclosure, this voltage will be referred to as the let-through voltage. It is the let-through voltage and speed that are the true measures of the effectiveness of any device or combination of devices used to protect electronic equipment. The lower the let-through voltage and the faster the response time, the better and more effective the protecting device is in the real world.
In practice, when the suppression device breaks down to the clamping mode of operation, there is a voltage across the device and the lines protected that can be as high as several hundred volts. This is the voltage "let-through" of the suppressor.
One reason for this relatively high let-through voltage is because the theoretical performance of the suppression device is compromised in the real world by the leads used to connect the device to the line conductors associated with the device or to connect the various component parts of the suppressor device together among themselves. For example, the suppression voltage of a given device may be two hundred volts at a certain current. However, the lead resistance and inductance to couple from the active element into the circuit of interest may add another two hundred volts of series voltage as well as increase the time required for the device to activate. This reduces the overall effectiveness of the device from two hundred volts to four hundred volts of let-through voltage and may let through a damaging high speed leading edge of the pulse disturbance.
In the past, the leads used with such suppression devices are very often of very small diameter which adds to the series resistance and inductance. In fact, many MOV's and other surge suppressors are manufactured with leads such as #22 wire. The problems become even more severe at high levels of AC load current because the impedance of the source becomes even lower. For example, a one hundred amp commercial AC power source may use a conductor with a cross-sectional diameter of approximately 1/2 inch. The #20 wire used on an MOV will have a diameter of approximately 1/64 of an inch. Even if larger leads are attached to the MOV, they would typically be less than 1/8 inch in diameter to facilitate easy wiring. In all of these cases, the performance of the suppression device with regard to the let-through voltage and speed is severely compromised by the lead impedance of the suppressor being large as compared to the impedance of the main power leads.
FIG. 1 illustrates the problem associated with the prior art. Shown in FIG. 1 is a circuit 10 adapted to protect an electrical device, indicated by load 12, from the effects of electrical disturbances, such as the above-discussed surges, transients, and the like. For the purposes of this disclosure, all of these electrical disturbances will be referred to as "surges", but it is understood that such is a term of convenience and not a term of limitation. Thus, load 12 receives power from a source 14 via line conductors 18. Both the load 12 and the power source can be single or multi phase as necessary, with the single-phase configuration being shown in FIG. 1. The line conductors 18 have an impedance associated therewith, and this impedance is indicated in FIG. 1 by blocks 20. As those skilled in the art understand, the impedance associated with a line conductor, such as line conductors 18, is distributed throughout the length of those line conductors, and therefore the blocks 20 are merely representations of such impedance. For the sake of simplicity, in the following discussion, the term "resistance" may be used in place of the more correct term "impedance" which accounts for frequency and energy storage dependent characteristics. However, it is to be clearly understood that whenever the term "resistance" is used in the following disclosure, claims and figures, this term is intended to imply and include the term "impedance" as well and to thus include all definitions, principles, characteristics, and the like associated with the term "impedance" by those skilled in the art. Of course, those skilled in the art will also understand that the term "resistance" as used herein will also include the principles etc. associated with the term "dynamic resistance" where appropriate. Also, for the sake of convenience, each block 20 is considered as having one-half of the total line impedance associated with the line. In the instant case, this is one-half Z1. The power source 14 establishes a voltage across the line conductors 18 to power the load 12.
As above discussed, certain conditions create an electrical disturbance across line conductors 18. This electrical disturbance is indicated in FIG. 1 as surge voltage V1. As was also discussed above, some sort of surge protection means is included in the circuit. This surge protection means is indicated in FIG. 1 as clamping element 22. This clamping element 22 is shown in FIG. 1 as being an MOV, but can be any other of the known clamping elements, or a combination thereof (hybrid). The clamping element 22 has a steady state configuration of an open circuit; however, upon the occurrence of a surge voltage V1, the clamping element 22 breaks down to a voltage Vs which is less than the surge voltage V1. The breakdown voltage can (theoretically) be zero (a short circuit) or can be any finite value suitable for the particular load, source and line conductors associated therewith.
As indicated in FIG. 1, and as discussed above, the prior art connects the clamping element 22 to the line conductors by means of leads 26. In fact, some manufacturers of prior art devices deliberately add leads to the protection device for connecting that device to the line conductors. These additional leads may be used by the manufacturer for purposes such as a means for protecting against overheating, as a means to extend the life of a device which uses less expensive components, or the like. As discussed above, these leads have a resistance and inductance associated with them. This impedance is indicated in FIG. 1 by the blocks 28, and as also discussed above, this impedance 28 can be quite high. For convenience, the impedances associated with the leads 26 are indicated in FIG. 1 by 1/2 Z1 and 1/2 Z2, respectively. It is observed that the inductive component can be treated in a similar manner, yielding considerably higher impedance at high frequencies, such as occurs at the leading edges of surges. This further degrades performance at high frequencies, and degrades total "speed" of protection provided by the suppressor element.
The voltage applied to load 12 as a result of the surge voltage V1 is indicated in FIG. 1 as V2. Voltage V2 is the let-through voltage associated with the surge voltage V1 and the particular clamping element 22 in the circuit 10. For simplicity, load impedance is not considered, and the term "R" is used in the following relationships. This let-through voltage is approximated by the relationship: V2.congruent.Vs+V1[R2/(R1+R2)]. Since Vs represents the design clamping voltage of the clamping element, and V2 represents the let-through voltage, the term [R2/(R1+R2)] could be visualized as being a figure of merit to determine how closely the real-world performance of a particular device approaches the design, or ideal, performance of a clamping element. That is, upon the occurrence of a surge, the figure of merit can be used to determine how close the voltage actually applied (i.e., the let through voltage) to a load protected by a particular device approaches the clamping voltage of that particular device. The lower the value of R2, the closer the let-through voltage V2 approaches the clamping voltage Vs. Even if the clamping element were to short (thus making Vs zero), with the prior art circuit 10, the let-through voltage V2 will still have a finite value above that of the design clamping voltage due to the presence of R2 associated with the leads 26, and this let-through voltage can be quite high if the surge voltage V1 is high. Thus, in the prior art surge protection circuits, the load 12 is still subject to high surge voltages and high frequency components even in spite of the presence of the clamping element 22. This is especially true if the leads 26 are long and small in diameter and hence have a high resistance under surge conditions.
However, even if a designer accepts the fact that there will be a let-through voltage greater than the design clamping voltage associated with the suppression device, his problems are not over. The small leads associated with presently available suppression devices create uncertainty in the level of let-through voltage. For example, the resistivity and ampacity of such leads may change with temperature, skin effects may create variations in impedance depending on the level of current, frequency components of the surge, propagation delays, and the like.
Still further, the leads may be subject to breaking at high temperatures and may thus impose a maximum power rating on the overall device that is lower than the rating of the device itself. This problem is important in lightning or Nuclear Pulse related surges which can be extremely high and can be accompanied by heating of associated components. Obviously, a broken lead to a suppression device may be disastrous since it opens the circuit which should be shunting thereby placing the full potential across the protected device.
The leads may also create a delay time in the operation of the suppression device. Advanced integrated circuitry may be subject to damage if a delay of as short as a fraction of a microsecond is present between the initiation of a transient and activation of the surge suppression device.
Therefore, the leads associated with presently available suppression devices create many problems, both to the designer and to the user of such devices.
Accordingly, any construction techniques which may minimize suppressor lead length are of value. Printed circuit board construction may often be used in a surge suppressor. However, virtually any length of conductor path generates an R2 value much greater than the R1 AC power source. Most printed circuit board applications will also use a suppressor device with the smaller diameter wire size. Also, printed circuit boards tend to be a weak point in high power situations. Other construction techniques may tend to use fixture wiring to connect the suppressor device to the AC power source. However, easy-to-handle wire sizes such as #14 to #12 still represent an appreciable R2 in the practical case since their lengths may range from a few inches to many feet.
Thus, there is need in the art for a surge protection means that protects against any or all electrical disturbances and which quickly reduces the let-through voltage to at or very near the theoretical value of clamping voltage associated with the clamping element itself so that the real world performance of such means approaches very nearly that of the theoretical clamping voltage of the clamping element itself, and at the highest speed and frequency components possible.